1. Field of the Invention
This invention relates to high-temperature ionic conductors for solid oxide fuel cells and more particularly to a class of ionic conductors stable at temperatures in the order of 600.degree.-800.degree. C. and which are based on framework structures with net positive or negative charges along channels, tunnels or planes that are large enough to transport an oxide ion or a hydrated proton.
2. Background of the Invention
Solid oxide fuel cells (SOFC's) can become one of the most durable and economical fuel systems for utility and transportation applications. Using solid electrolytes virtually eliminates corrosion reactions and electrolyte losses that are common in liquid electrolyte fuel cells. Furthermore, fuel processing for SOFC's is simpler and less expensive than other types of fuel cells.
Presently, SOFC's operate at temperatures of approximately 1000.degree. C. The requirement of high-operating temperatures to attain adequate conductivity levels limits the number of materials available for SOFC fabrication as most materials become compromised thermally, chemically and mechanically under these high temperature conditions. For example, the conductivity of the commonly used yttrium-stabilized zirconium oxide is 10.sup.-1 ohm.sup.-1 cm.sup.-1 at 1000 .degree. C. This conductivity decreases to 4.times.10.sup.-2 ohm.sup.-1 cm.sup.-1 at 800.degree. C. Examples of yttria-stabilized zirconia electrolyte use at high temperatures can be found in U.S. Pat. Nos. 4,476,196; 4,476,197 and 4,476,198, wherein the electrolytes facilitate ion transfer in electrochemical fuel cells operating in temperatures exceeding 1000.degree. C. As with the above-mentioned teachings, most fuel cells incorporating yttria-stabilized zirconia also rely on standard materials, such as zirconium-based cermet as constituents for the accompanying electrodes.
Presently known high-temperature electrolytes are oxide ion conductors that transport oxide ions by the vacancy migration mechanism. In the yttrium-stabilized zirconium oxide system, a positive charge deficiency is created by substituting some trivalent yttrium ions for the tetravalent zirconium ions in the cation sublattice. To compensate for the positive charge deficiency, oxide ion vacancies are formed in the oxide sublattice. These vacancies provide the stopping-off points for hopping oxide ions. Aside from zirconium oxide, other presently known oxide ion conductors include CeO.sub.2, ThO.sub.2, HfO.sub.2, and Bi.sub.2 O.sub.3. All of these host oxides contain various types of dopants to enhance conductivity. When these materials crystallize in the fluorite structure, oxygen ion vacancies can be found in the oxygen sublattice. These vacancies facilitate the mechanism for the hopping of oxides across the electrolyte thereby serving as the conduit for oxide ions through the electrolyte.
Operating a SOFC at more moderate temperatures, such as 600.degree.-800.degree. C., would allow much greater flexibility in engineering the fuel stack because metals could be used as interconnect and gasket materials. This would ultimately reduce the cost and open up new applications. With the present technology, it is not possible to lower the operating temperature of the fuel cell because the electrical resistance of the electrolyte increases exponentially as temperature decreases. To decrease the operating temperature, a new electrolyte is required.
New electrolytes have been discovered to conduct by a different mechanism; i.e. by transport of interstitial ions instead of by vacancy migration. These oxides do not crystallize in the fluorite structure. They have framework structures which feature channels or planes that are large enough to transport an oxide ion or a hydrated proton through them. By creating net positive or negative charges on the framework, interstitial oxide ions (such as O.sup.2-) or hydrated protons (such as H.sub.3 O.sup.30) are able to pass through the channels and/or planes at a high rate.